discount.php

<?php
    public function couponDiscount($cart)
    {
        $discount = 100; //test value

        if ($discount > 0) {
            // TODO: fix tax exclusion
            $cart->add_fee(__("Custom discount"), -$discount);
        }
    }

    add_action('woocommerce_cart_calculate_fees', [$this, 'couponDiscount'], 10, 1);